1. Field of the Invention
The present invention relates to a superconducting magnet with a magnetic shield for generating a highly uniform magnetic field for use in a magnetic resonance diagnosis apparatus.
2. Description of the Related Art
In magnets for a magnetic resonance diagnosis apparatus, a high-intensity uniform magnetic field must be generated in an image-photographing space of an opening of the magnet. A conventional magnet presents a problem in that, when such a magnetic field is generated, the magnetic field leaks to the outside and exerts a bad influence on peripheral devices. Therefore, magnetic coils are enclosed by a ferromagnetic substance so as to shut off magnetic fields.
FIG. 2 is a cross-sectional view showing a magnet with a magnetic shield disclosed in, for example, Japanese Patent Laid-Open No. 63-281410. In FIG. 2, a superconducting field coil 1 wound around a reel (not shown) is housed in a cryostat 2. A magnetic shield 3 provided in such a manner as to enclose the cryostat 2 comprises a cylindrical yoke 4 and an end plate 5 having a central opening 6.
Since electromagnets for a magnetic resonance diagnosis apparatus are required to generate a highly uniform static magnetic field, the field coil 1 is usually composed of two or more divided coils and the arrangement thereof is determined in such a way that a highly uniform magnetic field can be obtained by taking the magnetic shield 3 into account. The field coil 1 and the magnetic shield 3 are arranged in such a manner so as to be positioned symmetrically with respect to the axial direction of the coil, because if they are unsymmetric with respect to the center O of the coil and the axial direction X of the coil, a gradient magnetic field is generated and the uniformity of the magnetic field decreases.
FIG. 3 is a cross-sectional view showing the placement of the coils of a superconducting magnet, for example, separated into six sections. The field coil 1 comprises end-section superconducting coils 7a and 7f, intermediate-section superconducting coils 7b and 7e, and central-section superconducting coils 7c and 7d. These superconducting coils 7a, 7b, 7c, 7d, 7e, and 7f are wound in a state in which they are placed in series. Next, an example of the connection of a conventional superconducting magnet apparatus is shown in FIG. 4. In FIG. 4, superconducting coils 7a, 7b, 7c, 7d, 7e, and 7f are successively wound in series. Protective diodes 8a, 8b, 8c, 8d, 8e, and 8f are respectively connected in parallel to these superconducting coils, as shown in FIG. 4. A persistent current switch 9 is connected between both ends of these serially-connected six superconducting coils, forming a closed circuit.
In such a superconducting magnet formed as described above, a current in the circuit circulates through the closed circuit passing through the persistent current switch 9 through the superconducting coils 7a, 7b, 7c, 7d, 7e, and 7f, and thus a persistent current condition is maintained. In such a state, no current flows through the superconducting coil protective diodes 8a, 8b, 8c, 8d, 8e, and 8f. The superconducting coils 7a, 7b, 7c, 7d, 7e, and 7f and the magnetic shield 3 are positioned symmetrically with each other, and no non-equilibrium electromagnetic force will be generated.
On the other hand, in superconducting magnets, a transition to a normal conducting state sometimes occurs in superconducting coils which is caused by thermal or electromagnetic disturbance applied to the superconducting coils. If a transition to a normal conducting state occurs, electrical resistance appears inside the superconducting coils and the current attenuates rapidly. At this time, a voltage generated by the electrical resistance and an inductance voltage induced by the attenuation of the current are generated in the coil in which a transition to a normal conducting state transition has been generated. A voltage caused by the current attenuation is also induced in coils in which no normal conduction transition has occurred. When the voltage generated due to these normal conduction transitions exceeds a turn-on voltage of protective diodes 8a, 8b, 8c, 8d, 8e, and 8f, the diodes are turned on, causing both ends of the superconducting coils to be shorted. This short-circuit enables a voltage generated in the superconducting coils to be suppressed, thereby preventing insulation breakdown.
For example, a case in which the end-section superconducting coil 7a makes a transition to a normal conducting state in FIG. 4 will be considered. Electrical resistance appears in the superconducting coil 7a and the current in the superconducting coil 7a attenuates rapidly. However, a current which circulates through the superconducting coils 7b, 7c, 7d, 7e, and 7f flows through the circuit passing through the persistent current switch 9 after passing through the protective element 8a. The current is hardly attenuated because the resistance inside the circuit is small, but on the contrary the current attenuation sometimes increases due to the induction by changes in the current of the superconducting coil 7a. Therefore, the current flowing through the superconducting coil 7a becomes smaller than the current flowing through superconducting coils 7b, 7c, 7d, 7e, and 7f if a transition to a normal conducting state occurs in the superconducting coil 7a. Consequently, the axial current distribution of the superconducting coil group becomes asymmetric with respect to the center of the magnetic shield. As a result, a non-equilibrium electromagnetic force is generated between the superconducting coils 7b, 7c, 7d, 7e, and 7f and the magnetic shield 3. Hence, a superconducting coil support construction (not shown) must have a strength which can withstand this non-equilibrium electromagnetic force. As a consequence, there arises a problem in that the apparatus is complex in construction and expensive.
As a method for preventing such a non-equilibrium electromagnetic force, the connection of FIG. 5 is used in the prior art. That is, among separately placed superconducting coils, superconducting coils 7a and 7f, 7b and 7e, and 7c and 7d, which are positioned symmetrically to each other, are connected in series, forming coil pairs. These superconducting coil pairs are each connected in series, and pairs of protective diodes 10a, 10b, and 10c are connected in parallel between both ends of each of the above-described superconducting coil pairs respectively, as shown. In this connection, even when a transition to a normal conducting state occurs in any one of the superconducting coils and protective diodes are turned on, since a coil pair in which two superconducting coils positioned in symmetry are connected in series are shorted by corresponding protective diodes, the currents of the superconducting coils positioned symmetrically are equal to each other and no equilibrium electromagnetic force will be generated. However, this method has drawbacks in that lead wires for connecting the coils are long because superconducting coils successively connected in series are spaced apart as coils positioned spatially symmetrically must be connected in series, and therefore, the wiring is intertwined and complex and the characteristics are liable to be unstable.